The present invention concerns power line communication systems and, more particularly, a novel method for selectively interrupting connection, in the vicinity of zero crossings of the network waveform, to loads connected thereto to provide a high impedance condition for enabling transmitters and rereceivers of the carrier communication system to be efficiently coupled to the network for improved communications reliability.
It is well known that information may be communicated between different points on a network of conductors, such as the commercial power distribution network and the like. As an example, in a power line carrier (PLC) communications system, a carrier signal, at a carrier frequency several orders of magnitude greater than the power line frequency (60 Hz. in the United States), may be transmitted in either direction between individual locations (such as a residence, office building, manufacturing plant and the like) and a central facility (such as a power company substation main distribution transformer location and the like). To assure reliable communications, it is desired that as high a signal-to-noise ratio be present for the transmitted carrier, at any carrier-receiving location. However, such PLC signals will generally be required to be transmitted through at least one transformer, typically the distribution transformer serving to reduce the network voltage, which may be on the order of 13.2 KV., to the distribution voltage, which may be on the order of 120 V. Such transformers typically have a leakage reactance deliberately designed so as to limit short-circuit current in the distribution transformer in the event of a secondary circuit fault. This leakage reactance provides an impedance, in series with the transformer secondary winding, which is typically equal to 5-10% of the nominal full load impedance at the power line frequency. At frequencies utilized for PLC communications, the impedance of this leakage reactance is even larger than the power line frequency impedance thereof and tends to greatly attenuate the PLC carrier waveform, thereby reducing the signal-to-noise ratio at the PLC receiver. A method for reducing the amount of attenuation, due to coupling through a distribution transformer, of PLC communications signals, and without the use of a capacitor bypass across the distribution transformer or the use of apparatus for repeating the signal around the transformer, is highly desirable.